1. Field of This Invention
This invention relates to an activated catalyst for oxidation reactions, such catalyst containing a mixed oxide of vanadium and phosphorus, a process for the production of the activated catalyst and a process for using the activated catalyst for producing maleic anhydride from straight-chain C.sub.4 -hdyrocarbons in the gaseous state.
2. Prior Art
Oxidation catalysts based on mixed oxide of vanadium and phosphorus, which can also contain oxides of other elements, such as, TiO.sub.2, are known. See German Patent Nos. 2,256,909 and 2,328,755. Such catalysts are particularly useful for the selective oxidation of butane into maleic anhydride--see U.S. Pat. No. 3,293,268.
According to German Patent No. 2,256,909 the catalyst is brought to an average (median) valence for the vanadium of preferably +4.1 to +4.5 by a complicated step by step activation in various gases and gas mixtures, for example, with oxygen containing hydrocarbons. The optimal value for the valence of a catalyst, activated in such a manner, lies at +4.2. So Table IV of the English Patent No. 1,416,099, which corresponds to German Patent No. 2,256,909, shows that the yields which are achieved with a catalyst, wherein the median valence of the vanadium lies at +3.9, drop considerably, and, at a conversion of 39 percent, the yield is only 23 percent by weight.
It is also known that such catalysts quickly become inactive. To achieve reactivation, such catalysts are treated with reduction agents, such as, H.sub.2, CO, CH.sub.4 and H.sub.2 S, at a temperature of about 500.degree. C. With such treatment one achieves compensation for the deactivation brought about by over oxidation of the vanadium which reduces the catalyst activity. By means of the reduction treatment, the average valence of the vanadium is adjusted to +4.2 to from +4.6--see German Patent No. 2,353,136.
Such catalysts, even if they are reactivated, achieve yields of at most about 75 percent by weight in the case of oxidation of butane to maleic anhydride--see German Patent No. 2,353,136.